Five hundred (500) patients with acute coronary syndrome undergoing PCI for a culprit lesion are to be enrolled in the study to undergo coronary vascular profiling at the time of the index catheterization procedure. Up to 374 consecutive patients with at least one low ESS subsegment are to have follow-up coronary angiography and IVUS at 6-10 months to allow for at least 300 patients with analyzable intracoronary vascular profiling data for assessment of lesion natural history. All patients are to have a one-year clinical follow-up to assess for new cardiac events, followed by two additional years of extended clinical followup.